Hm.. it seems this functionality IS ACTUALLY ALREADY built into VLC.
What did I do to find this?
I downloaded Audacity + the ffmpeg im/export lib (get it to read the video containers) and looked at the wave-form display..
ACDC - Thundstruck (most of the files wave-forms I got look like this):
Puretone - Addicted to Bass (quiet files look like this):
I then looked around Audacity for means to amplify the Puretone track to the same level as the ACDC one.. easy: [Effect > Amplify] brings up a dialogue which suggests a gain of +13.5dB for the whole track (without clipping) and personally I did went for +16dB to make it look like this:
Back in VLC with this new knowledge I played around with the audio settings in the preferences to get similar results:
[Replay Gain mode] > Track
[Replay Preamp] > 16.0
Now playing the files of different loudness actually amplifies the quieter ones up to the same output level as the louder ones and I'm happy.